The present invention relates to a method for enhancing the recovery of petroleum from an oil-bearing formation. In particular, the invention relates to the use of a mixture of anionic surfactants, cationic surfactants and optionally, nonionic surfactants, in petroleum formations.
In the recovery of light oils (i.e., greater than 20.degree. API) from reservoirs, the use of primary production techniques (i.e., use of only the initial formation energy to recover the crude oil), followed by the secondary technique of waterflooding, recovers only about 60 to 70% of the original oil present in the formation.
Moreover, the use of certain enhanced oil recovery (EOR) techniques is also known within the art. These techniques can be generally classified as either a thermally based recovery method, i.e., utilizing steam, or a gas-drive method that can be operated under either miscible or non-miscible conditions.
The gases which are commonly employed in gas-drive methods include for example, nitrogen, carbon dioxide, methane, mixtures of methane with ethane, propane, butane, and higher hydrocarbon homologues. This class of gases includes both natural gas and produced gas.
A typical procedure involves injecting a slug of CO.sub.2 followed by the injection of a higher viscosity fluid such as water to "push" the CO.sub.2. See, for example, the discussion in U.S. Pat. No. 2,623,596. Moreover, U.S. Pat. No. 3,065,790 indicates that this process may be more cost effectively employed if a relatively small slug of CO.sub.2 is injected ahead of a drive fluid. In fact, as illustrated by U.S. Pat. No. 3,529,668, this type of recovery procedure is typically performed in "water alternating gas (WAG)" cycles.
Because of the viscosity and density differences between the CO.sub.2 and the oil (i.e., CO.sub.2 has only 5 to 10% of the viscosity of ,e.g., light oil), the CO.sub.2 tends to bypass much of the oil when flowing through the pores of the reservoir rock.
One proposed solution to this problem associated with the bypassing of the CO.sub.2 has been through the injection of water which contains a surfactant, with the CO.sub.2. In particular, a surfactant has been proposed as a means for generating a foam or an emulsion in the formation. See, for example, U.S. Pat. No. 4,380,266 to Wellington and U.S. Pat. No. 5,502,538 to Wellington et al.
The purpose of this foam is to inhibit the flow of the CO.sub.2 into that portion of the formation containing only residual oil saturation. In addition, the foam physically blocks the channels through which CO.sub.2 is short-cutting.
When employing gases, the creation of an effective foam is very difficult because either the salt concentration of the water in the formation (connate or injected brine), the residual oil in the reservoir, or the chemical instability of surfactants tends to break the foam or even prevent the foam from forming.
One environment where the formation of a foam is particularly troublesome is that where the rock surfaces tend not be wetted by water, i.e., oil is instead adsorbed onto the rock surface, which also destabilize foams. These types of rocks are known as "oil-wet".
The need still exists for a surfactant composition which can be effectively employed in oil-wet environments, e.g., oil-wet sandstone, limestone and/or dolomite formations.
Furthermore, although both cationic and anionic surfactants are individually recognized in the art, they are not traditionally employed in combination because, when mixed, they form a salt complex which precipitates out of such a mixture, i.e., the two charged compounds combine to form a complex having decreased solubility.